The new approach to cannabis possession began in July
Scotland Yard said the rise in muggings in Lambeth coincided with a rise in street crime across London.
But shadow home secretary Ann Widdecombe said there was probably a connection between the rise in crime in Lambeth and the new approach to cannabis.
"If you send out the message that you are soft-peddling on policing it will have that effect.
"Also, a lot of street robberies are drug-related - I suspect if people think it's a soft area for drugs it's where they are going to carry out robberies."
Lambeth police chief, Commander Brian Paddick, launched the new policing approach to cannabis in July as a six-month pilot scheme.
It was seen as a necessary result of police officers' heavy workload.
Home Secretary David Blunkett had expressed interest in the scheme because it fitted in with the government's emphasis on tackling hard drugs.
Police deny link
But in the month the scheme was introduced muggings in Lambeth continued to rise.
Scotland Yard figures showed there were 760 street robberies and snatches in July, up from 613 the previous month and from 489 in July 2000.
Across London there were 5,427 such incidents last month, 175 incidents a day.
It was the third consecutive monthly rise for the whole of London with 4,860 incidents in April, 5,229 in May and 5,335 in June.
Police said there was no evidence of a link between the new approach to cannabis and the rise in street crime in Lambeth.